patch 9.1.1539: completion: messages don't respect 'shm' setting

patch 9.1.1539: completion: messages don't respect 'shm' setting

Problem:  completion: messages don't respect 'shm' setting
Solution: Turn off completion messages when 'shortmess' includes "c"
          (Girish Palya).

`:set shortmess+=c` is intended to reduce noise during completion by
suppressing messages.
Previously, some completion messages still appeared regardless of this setting.

This change ensures that **all** completion-related messages are suppressed
when `'c'` is present in `'shortmess'`.

Not entirely sure if the original behavior was intentional. If there's a
reason certain messages were always shown, feel free to close this without
merging.

closes: #17737

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>